1. Field of the Invention
The invention disclosed and taught herein relate generally to topsides for offshore structures and related installation methods and systems; and more specifically related installation methods and systems to preloading float-over barges to reduce loads and save steel on topsides and grillage of catamaran systems.
2. Description of the Related Art.
A Spar platform is a type of floating oil platform typically used in very deep waters and is among the largest offshore platforms in use. A Spar platform includes a large cylinder or hull supporting a typical rig topsides. The cylinder however does not extend all the way to the seafloor, but instead is moored by a number of mooring lines. Typically, about 90% of the Spar is underwater. The large cylinder serves to stabilize the platform in the water, and allows movement to absorb the force of potential high waves, storms or hurricanes. Low motions and a protected center well also provide an excellent configuration for deepwater operations. In addition to the hull, the Spar's three other major parts include the moorings, topsides, and risers. Spars typically rely on a traditional mooring system to maintain their position.
Deck or topsides installation has always been a challenge for floating structures, particularly in deep draft floaters like the Spar, which must be installed in relatively deep water. In the past heavy lifting vessels (“HLV”), including but not limited to, derrick barges have been used for topsides installations.
In traditional efforts, the topsides requires multi-lifting, for example five to seven lifts, to install the whole topsides due to the lifting capacity of available HLV. Due to multi-lifting, the steel weight per unity area of the topsides can be higher than that of topsides of fixed platforms installed with a single lifting. If the weight of the topsides is reduced, the weight of the Spar hull may also be reduced. The same principles are applicable to other offshore structures to which a topsides can be mounted.
Recently catamaran float-over systems have been used to install a topsides onto a Spar platform. A float-over method is a concept for the installation of the topsides as a single integrated deck onto a Spar hull in which the topsides is first transferred from a single barge onto at least two float-over barges (called “offloading”) and transported with the float-over barges to the installation site for the Spar hull. At the installation site, the float-over barges are positioned on both sides of the Spar hull with the Spar hull below the topsides, the elevation is adjusted between the topsides and the Spar hull, and the topsides is installed to the Spar hull. Installation of the topsides to the Spar hull by the float-over method can allow a high proportion of the hook-up and pre-commissioning work to be completed onshore prior to load-out, which can significantly reduce both the duration and cost of the offshore commissioning phase. The float-over installation method allows for the installation of the integrated topsides or production deck on a fixed or floating structure without any heavy lift operation.
However, to accomplish the catamaran float-over procedure, the float-over barges are necessarily separated. During loading and transportation to the desired location for float-over and installation of a topsides on a Spar hull, the catamaran system is subjected to several loading conditions primarily due to wave action on the separated barges. These loading conditions would not occur with a single barge loaded with the topsides on deck, but such a single barge arrangement would not be conducive to a float-over installation of the topsides.
FIGS. 1A-1B illustrates two major different modes of loading. FIG. 1A is a schematic top view of a racking load on a catamaran system used to install topsides on a Spar hull. FIG. 1B is a schematic end view of a lateral bending load on the catamaran system. The figures will be described in conjunction with each other. In general, a catamaran system includes at least a pair of barges 115a, 115b (generally 115). A fabricated topsides 110 is removably coupled to the barges 115 through a supporting structure, referenced herein as a grillage system 125a, 125b (generally 125) mounted to the barges 115a, 115b, respectively. Different loads 101-102 occur on the catamaran system 100 that are not prevalent in a single barge system. These loads can include (i) racking moments 101a, 101b (generally 101), as shown in FIG. 1A, where the barges 115 are prone to twist relative to each other in response to wave loads causing stresses on the system; and (ii) lateral bending moments 102a, 102b (generally 102), as shown in FIG. 1B, where the barges 115 are prone to twist laterally in response to wave loads causing stresses on the system. The catamaran system 100 generally behaves as a rigid body when it is subjected to head and beam seas. Wave diffraction on single body catamaran system 100 has been performed to calculate the hydrodynamic load on this system.
To withstand these different loads particular to a catamaran system, the members used to construct the topsides and the grillage system are strengthened generally by an increase in size, adding weight and expense, compared to a single barge system with the topsides loaded onto the single barge. Because a topsides is generally a functioning micro-city suitable for extensive periods for working crews and other personnel, the topsides structure is relatively a significant size. An overall increase in size of even a small percentage can become a significant increase in actual expense.
There remains then a need to provide a catamaran system for a float-over procedure with a topsides, but more efficiently use the weight and strength of the members in the catamaran system to reduce weight and costs.